1. Field of the Invention
Embodiments of the present invention generally relate to methods for forming zinc oxide based thin film transistors.
2. Description of the Related Art
Current interest in thin film transistor (TFT) arrays is particularly high because these devices are used in liquid crystal active matrix displays of the kind often employed for computer and television flat panels. The liquid crystal active matrix displays may also contain light emitting diodes for back lighting. Further, organic light emitting diodes (OLEDs) have been used for active matrix displays, and these OLEDs require TFTs for addressing the activity of the displays.
The TFT arrays may be created on a flat substrate. The substrate may be a semiconductor substrate, or may be a transparent substrate such as glass, quartz, sapphire, or a clear plastic film. A gate dielectric layer overlies a gate electrode. A passivation dielectric overlies the upper surface of a second conductive electrode, to electrically isolate the second conductive electrode from the ambient surrounding the upper surface of the TFT device.
TFTs made with amorphous silicon have become the key components of the flat panel display industry. Unfortunately amorphous silicon does have its limitations such as light sensitivity, light degradation, and limited aperture ratio for active matrix arrays. Another problem associated with amorphous silicon is its low field effect mobility. The mobility required for OLEDs is at least 10 times higher than that achievable with amorphous silicon. In recent years, transparent TFTs have been created in which zinc oxide has been used as the active channel layer. Zinc oxide is a compound semiconductor that can be grown as a crystalline material at relatively low deposition temperatures on various substrates such as glasses and plastics. Zinc oxide based TFTs will not degrade upon exposure to visible light. Therefore, a shield layer, as is necessary for silicon based TFTs is not present. Without the shield layer, the TFT remains transparent.
In forming zinc oxide and doped zinc oxide for TFTs used in flat panel displays, it is difficult to effectively deposit the zinc oxide and doped zinc oxide while also forming an active channel. There is a need in the art for effectively forming a zinc oxide based TFT.